Graphite based materials in particulate form are the principal materials used as carbon materials in negative electrodes of lithium ion secondary batteries. The raw materials of graphite based materials include artificial graphite powder and natural graphite powder. It is more common to use artificial graphite powder, but there is increasing use of natural graphite powder from the standpoint of achieving a high degree of economy.
The most important challenge in the development of a lithium ion secondary battery is increasing the capacity of the battery. With the object of meeting this challenge, there has been much research aimed at increasing the capacity per unit mass of a graphite based material. As a result of such efforts, a negative electrode material made from a graphite based material which has been developed can provide a negative electrode exhibiting a capacity exceeding 360 mAh/g compared to the theoretical capacity of graphite of 372 mAh/g, which is the maximum capacity which can theoretically be obtained by a negative electrode made from a negative electrode material composed entirely of graphite. Therefore, increases in the capacity of batteries having a negative electrode obtained using a graphite material as a negative electrode material by improving graphite based materials themselves have nearly reached a limit.
Under such circumstance, as a method of further increasing the capacity of a negative electrode, it has recently been attempted to increase the density of a negative electrode by compressing a negative electrode material made from a graphite based material. In order to increase the density of a negative electrode by compression of a negative electrode material, it is necessary for a graphite based material to deform and fill vacant spaces between adjacent particles when the graphite based material is compressed to form a negative electrode. Therefore, from the standpoint of increasing the electrode density, a soft graphite based material is preferred.
In this regard, graphite based materials made from highly crystalline graphite such as natural graphite (referred to below as highly crystalline graphite particles) are very soft because they easily develop interlayer sliding. Therefore, highly crystalline graphite particles readily deform. Accordingly, use of highly crystalline graphite particles as a raw material makes it possible to easily obtain a negative electrode material capable of providing a negative electrode with a high density.
However, due to the softness of highly crystalline graphite particles, the formation of a large number of closed pores occurs inside a negative electrode obtained by compressing a negative electrode material made solely from highly crystalline graphite particles. This caused the problem that the charge acceptance of the resulting negative electrode decreased.
Charge acceptance is an index of how smoothly a negative electrode material reacts with lithium ions. If it is low, precipitation of lithium metal takes place during charging. In the present invention, charge acceptance is defined by the charging capacity of a negative electrode, which is measured by the method described below in examples.
In order to overcome the problem of a decrease in charge acceptance, the following means have been employed.
(i) The surface of highly crystalline graphite particles is coated with low crystallinity carbon. The resulting graphite based material is referred to below as coated graphite particles.
(ii) Low crystallinity carbon is locally adhered to the surface of highly crystalline graphite particles. The resulting graphite based material is referred to below as adhered graphite particles.
The low crystallinity carbon material present on the surface of coated graphite particles and adhered graphite particles is very hard, and hence both of these graphite based materials have a high overall hardness. Therefore, interlayer sliding is suppressed in highly crystalline graphite particles inside a graphite based material. Accordingly, with a negative electrode obtained by compressing a negative electrode material made of coated graphite particles and/or adhered graphite particles, the formation of closed pores in the interior of the negative electrode is suppressed, and as a result, a decrease in the charge acceptance of the negative electrode is suppressed.
However, in this case, since the graphite based material which constitutes the negative electrode material is very hard, it is necessary to increase the compressive force applied to the negative electrode material which is compressed in order to obtain a negative electrode. Therefore, when it is not possible to adequately compress the negative electrode material due to equipment limitations, for example, it is not possible to increase the density of the negative electrode.
Even when there are no equipment limitations and it is possible to adequately compress a negative electrode material, if a negative electrode material containing coated graphite particles and/or adhered graphite particles is excessively compressed in order to obtain a high density negative electrode, marked crushing of the hard carbon material present on the surface of the coated graphite particles and/or adhered graphite particles occurs. Crushing of the carbon material on the surface forms a large number of new surfaces on the negative electrode material constituting the negative electrode, and a SEI (solid electrolyte interface) film is formed on the newly formed surfaces. This SEI film causes the problem of an increase in the irreversible capacity (charging capacity minus discharge capacity) of a battery.
Concerning this problem, Patent Document 1 discloses mixing flake graphite particles with coated graphite particles having their surface coated with a hard carbon material made from amorphous carbon. The flake graphite particles are easily crushed and act as a cushion. Therefore, incorporation of the flake graphite particles suppresses crushing of coated graphite particles during rolling of a negative electrode, thereby suppressing an increase in specific surface area. It is disclosed in that document that a decrease in charge and discharge efficiency is thereby suppressed.
Patent Document 2 discloses mixing coated graphite particles with a graphite based material obtained by heat treatment at a high temperature of graphite particles prepared by spheroidizing pulverization of natural flake graphite (see claim 4 and Example 6, for example). Patent Documents 3 and 4 propose mixing coated graphite particles with uncoated graphite particles.
Patent Document 5 discloses maintaining pores by mixing uncoated graphite particles having a large particle diameter with hard coated graphite particles having a small particle diameter. Patent Document 6 discloses mixing hard coated graphite particles which are coated with amorphous carbon or turbostratic carbon with uncoated graphite particles.